Many yaw-rate sensors are based on the utilization of the Coriolis effect as a measuring principle. The Coriolis force occurs when a body of mass m moves at velocity v and a rate of rotation Ω acts in a direction perpendicular to the direction of movement, whereFCoriolis=2mvΩ.
A possibility for setting a mass into motion is to induce a vibration in it, the driving vibration. If a rate of rotation acts on the vibrating mass, then the Coriolis forces cause it to react with a Coriolis vibration perpendicular to the driving vibration.
If the mechanism is combined with the electronics to form a resonant circuit, then a resonator is formed and the frequency of this driving vibration corresponds to the resonant driving frequency f_rA of the mechanism. The Coriolis vibration then occurs at the resonant driving frequency, as well. However, resonant frequency f_rD of the Coriolis mode is independent of resonant driving frequency f_rA. Since the sensitivity to the Coriolis effect is greatest at the resonant frequency of Coriolis mode f_rD, it is useful to adjust frequency f_rD to correspond to resonant driving frequency f_rA.
This adjustment of resonant Coriolis frequency f_rD may be accomplished with the aid of electrostatic, positive-feedback forces, which counteract the mechanical spring stiffness and therefore lead to an effective reduction in the spring stiffness, through which resonant frequency f_rD may be reduced. An option for adjusting the frequency is to design the mechanism in such manner that its resonant Coriolis frequency f_rD is greater than the resonant driving frequency. Then, by applying an electrical voltage U_DF to the sensor element, the electronics may generate electrostatic, positive-feedback forces so that the resonant frequency of the Coriolis mode is reduced sharply enough to correspond to the resonant driving frequency. Electrical voltage U_DF_Abgl required for this purpose may be adjusted inside the electronics. This is described, for example, in German Patent Application No. DE 19910415 A1.
One problem with the conventional system occurs when further voltages must be generated in the electronics for the purpose of measuring or compensation; those voltages also acting on the sensor element as well, and shifting resonant Coriolis frequency f_rD to lower frequencies, due to the electrostatic positive feedback caused by the voltages. All of the voltages, which also act on the sensor element for reasons of measuring or compensation, are represented by voltage U_MK in the following analyses. When adjusting voltage U_DF_Abgl, one must make sure that U_MK assumes a value that is typical for the operation of the sensor. This value is hereinafter referred to as U_MK_Abgl.
During operation of the sensor, U_MK may also change by the amount of voltage difference ΔU_MK. The reasons for this could be, for example, that circuits for suppressing interference in the sensor element change their output voltage due to temperature dependences or long-term drift. Voltage change ΔU_MK leads to a corresponding, disruptive shift of resonant Coriolis frequency f_rD.